Mechanical Properties and Kinetics of Borided AISI M50 Bearing Steel

Abstract

The present study reports on mechanical properties and kinetics of borided AISI M50 steel. AISI M50 steel is widely used for critical aircraft engine bearings operating at elevated temperatures with its combination of good hot hardness and excellent rolling contact fatigue strength. AISI M50 is also intermediate high-speed steel which make it suitable for using as woodworking tools and hacksaw blades. Despite its wide range of use, there is almost no published research on the boronizing of AISI M50 steel in the literatures. Boronizing thermochemical treatment was carried out in a solid medium consisting of EKabor-I powders at 850, 950, and 1,050 °C for 2, 4, 6, and 8 h, respectively. The presence of borides FeB and Fe2B on the steel substrate was confirmed via X-ray diffraction analyses and energy-dispersive X-ray spectroscopy. The results of this study indicated that the boride layer has a smooth and compact morphology, and its hardness was found to be in the range of 1,630–2,040 HV. Depending on process time and temperature, the thickness of boride layer ranged from 6 to 216μm. Layer-growth kinetics were analyzed by measuring the extent of penetration of the FeB and Fe2B sublayers as a function of boronizing time and temperature. The fracture toughness of borides ranged from 3.89 to 3.51 MPa m1/2. Moreover, an attempt was made to investigate the possibility of predicting the iso-thickness of boride layer variation and to establish an empirical relationship between process parameters and boride layer thickness.